Embedded Files
...at A.R. Johnson High School, Richmond County, GA
John Engelbrecht Grovetown Electrical Engineer johnenge@earthlink.net March 3, 2015
A Proposal for A.R. Johnson HS CTAE Career Day 2016,
in one of the classrooms, on the engineering side
A large speaker, 15", demonstrating how the cone produces sound, from .3 Hz to 200 Hz.
Objective supporting Career Day: several features of the demo show engineering in action:
practical
economical
using science & math as tools
optimizing an effect that has benefit for production or the consumer
safety: no shock, no fire
Poster details the engineering features.
Side benefit: gets the attention of even 8th graders. Most students have not seen vibrating speaker cones & only roughly know that cone vibration produces sound.
The speaker is mounted in a corrugated cardboard temporary door*, set in the classroom doorway so that the classroom constitutes an "infinite baffle," with excellent acoustic properties. The speaker cone is open to view & illuminated so motion is obvious, rather than being sealed in a cabinet. Student research option, often via Wikipedia: infinite baffle.
Sound sources: 1) IASCA Official Sound Quality Reference CD, 2) sine wave source that Mr. Engelbrecht brings, 3) CDs brought by students from home, 4) microphone. Student research option: IASCA.
Complemented by smaller speaker set, inside the classroom, above 120 Hz or so, to fill out the sound spectrum.
High-power amplifier is home-built, transistors are oil-immersion cooled rather than fan cooled as a novelty. Power supplies are commercial 24V 8.3 amp or so, nice and safe. All expenses are covered by Mr. Engelbrecht who takes equipment home with him at end of day. Student research option: power transistor, also Mr. E brings a cut-open transistor. Also seen as the Darlington transistor at http://en.wikipedia.org/wiki/Transistor.
A possibility: Mr. Engelbrecht works with students to build some of the setup, after Mr. E gets volunteer training. Dar___ Fa____ has e-mailed Mr. E asking to participate in extracurricular learning, as Mr. E's handout in Rm. 808 on Career Day offered. This handout appears on https://sites.google.com/site/solderandcircuits/home/teachers--topics-in-common-with-your-teaching. If students build up some of the demo, students can conduct the demo.
Acknowledgement to be made that Class AB amplifiers are being obsoleted by Class D, which are more efficient.
Photos on Promethean board of how speakers are really used, in cabinets.
A bare speaker produces little sound, you have to capture the sound from the back of the cone & keep it from being audible. (This can be part of the demo.) Cabinets even have to be leakproof to avoid whistling. Student research option: http://en.wikipedia.org/wiki/Loudspeaker_enclosure
Small home & PA enclosures cause compromise in sound; lack of bass.
Theaters have big enclosures. (Possible field trip, & another is to Anything Audio.)
Music concerts generally have large speakers owned by the group or rented.
This proposal follows along after https://sites.google.com/site/solderandcircuits/example-of-homemade-sound-system
* Side effect of speaker-in-a-doorway: half the sound goes out into the hall. To minimize the distraction, the big speaker can be limited to 120 Hz max.
This is not the new 15" speaker, this is a burned-out speaker I found years ago. The voice coil is 2" whereas the JL speaker below is 2.5". More expensive speakers reach 4" in the 15" speaker size.
The dark hole with the screen over it is common to large speakers, This is what it is for: there is a dust cap over the top of the voice coil, glued onto the cone. When the voice coil pumps up & down, there would be so much pressure buildup that the speaker wouldn't be efficient; the hole is where the trapped air pushes out. The screen is to keep steel screws or debris out of the magnet gap. The magnet gap has intense magnetism, from the 8 pound ferrite magnet; if you put a paper clip around the gap, it really gets pulled.
The clearance within the gap for the coil isn't much, maybe .003", very tiny. The bigger the clearance, the less the magnetic flux, & the less efficient the speaker. Speakers are precision items, whatever their size.
This was apparently a 2ohm speaker. The wire has so much current going through it that it does get hot, just like motors get hot. There is very little air flow over the coil. In fact, a speaker is a motor, but it is a linear motor, not a rotating motor.
Tweeter coils are much smaller & prone to burnout.
March 9, 2015 I purchased the only 15" speaker that Anything Audio (Washington Road) stocks, JL Audio 15W0v3, a subwoofer, $225, plus two 6x9" car speakers Audiovox BU69.3 at $15 each, which will be paralleled or series with a small power amplifier with a built-in high-pass filter to produce midrange sound above the 80Hz (recommended by the man at Anything Audio), 120Hz, or so range of the JL subwoofer. (The 15" speaker's large power amplifier will also have built-in crossover, a low-pass filter.) Student research option: http://en.wikipedia.org/wiki/Audio_crossover
The subwoofer has a measured (by me, on March 7) D.C. cone travel of 6mm per 2.8A, & the speced Xmax is 12mm each way, so peak current might be 5.6A. For RDC of 3.55 ohms, times maybe 1.15 when the voice coil is hot, plus maybe .3 ohm of output-transistor emitter R, this is only a peak of 24.5V, or for a bridged amplifier that is 12.3V (peak +12.3V onto left of speaker, -12.3V onto right). Student research option: http://en.wikipedia.org/wiki/Bridged_amplifier#
Even if coil R is 15% high from heating, this is only 64W RMS, compared to JL's "optimum" suggestion of 270W. I puzzled for half a day why there is such a discrepancy. Part of it is that the typical user wants to push so much current that there will be some distortion (in other words, Xmax will be exceeded). But I figured out that, probably, the big factor is that the normal use of this speaker is in a 16"x16"x18" (small) sealed encloure, & that tiny box puts up so much counterpressure that the cone needs a lot more current than 5.6A to reach Xmax. Student research option: http://en.wikipedia.org/wiki/Voice_coil
The wires to the voice coil are not floppy, they are sewn into the spider.
This heavy speaker is 15.8 pounds. Much of that is a ceramic (ferrite) magnet. The deepness of this speaker is partly due to JL's Elevated Frame Cooling.
Despite being designed for a tiny, sealed enclosure, the Qts is a relatively high .482. The suspension does seem to be pretty stiff. Free-air resonance is 24.4Hz. I don't know how this changes when the speaker is in a room-size infinite baffle (mounted in a cardboard panel set in a doorway.) Intention is to add a bandstop filter to compensate for the 24.4Hz cone resonance, and give low-freq amplifier response down to .3Hz. Student research option: http://www.eminence.com/support/understanding-loudspeaker-data/
When you put less than 20Hz, at high current, onto this speaker, the Elevated Frame Cooling isn't going to cool too much because the cone will be moving so slowly. I posed a question to JL through their knowledge base, about moving +-12mm at 1Hz, and got a prompt answer! My question was whether a 12V fan could be added behind the hole in the magnet, seen in the photo just above, & pull cooling air through the coil when freq is low. I don't know that the responder really understood, because his response was,
"Eric Cole (JL Audio) Mar 9, 10:00 Hello, the vent on the back or bottom is actually where the hot air is pumped out. It may be more effective to use cross flow fans near the top of the magnet to allow cool air to be forced into the elevated frame area. Sincerely, Eric Cole JL Audio Technical Support." Eric's response gives the idea that the pumping action pushes air out the big vent, one way, whether the cone is going out or going in, which isn't what it would do. So I am uncertain how to take his response. I am inclined to heat the voice coil with maybe 70W at D.C. for 2s, then go to zero current & promptly measure the coil resistance, to get an idea of temperature of the copper wire. After doing this initial evaluation, decide a D.C. value to use for 30s, & set a 12V fan on the cooling port to see if the air seems heated. As a matter of fact, in the top photo there is seen a gap between steel & black-plastic magnet cover. It wouldn't be hard to tape a lightweight baffle onto 180 degrees of the gap & suck air through there, which I guess is what Eric was talking about. I wish I knew what is under that decorative, black-plastic cover.
Selection of commercial, open-frame, switching power supplies: Jameco 668730 24V, 8.3A, 200W, $55 each, qty two. This supply has good line & load regulation. The subwoofer current, 6.5A peak, plus an amp or so for midrange, is covered by these supplies. With well-defined supplies, rather than a range like 23V to 30V, the amplifier circuits will be a little more economical & efficient.
Frequency range: above 7kHz isn't too audible, intentionally limit the high end to 7kHz. At low end, try for .3Hz so subwoofer cone movement can be seen. Note: there is not too much instrumental content, other than drums, below 50 Hz, but the IASCA (International Auto Sound Challenge Association) Official Sound Quality Reference CD has prominent 37Hz piano on track 5. Student research option: http://en.wikipedia.org/wiki/Hi_fi
The requirements to protect the speakers, protect against radio-frequency interference (RFI), and provide flexible controls for demonstration to students are complex enough that plans are needed. See the two tables below. This plan, if carried to completion, will produce the most flexible sound-system demonstrator in the CSRA. There will be a control panel with numerous switches and volume controls to put the system through it's paces, demonstrating various aspects of speaker-system design. Protection against RFI (mainly FM radio stations and cell phones) is advised by the book, Designing Audio Power Amplifiers by electrical engineer Cordell, McGraw-Hill, 2011. Note: Mr. Engelbrecht has built audio amplifiers that were susceptible to FM radio station interference, it is a sad thing to go to the trouble of building an amplifier and then have a radio station coming out of the speaker.
Custom Audio Amplifier Outline
Read the table from left to right.
Notes
The single signal at J/Woof splits out to the two channels, the woofer amplifier and the midrange amplifier. The soft clipper might have an LED warning that clipping is happening.
The single input at Input/Woof can come from J/MIc, J/Sine, a CD player, the audio output of a computer, etc. This input is a two-input, differential transistor amplifier with neither input being ground; this rejects hum and buzz that might result from a CD player being plugged into one AC outlet and the DC Supplies being plugged into a different AC outlet. Student research option: http://en.wikipedia.org/wiki/Ground_loop_(electricity) LED at F/Woof turns on when Vpeak is .3V, as an aid to getting undistorted speaker signal; gain from .3V peak to +-12.3V peak at speaker terminals (bridged) sets the subwoofer amplifier gain at 12.3/.3=41.
EMI filters are RF (radio frequency) filters to keep AM & FM radio, and cell phone signals, out of the audio circuits; any RF getting in can be demodulated by transistor junctions, and will then be heard in the speakers, a serious defect in an amplifier. Student research option: http://en.wikipedia.org/wiki/Electromagnetic_interference, subheading RF immunity and testing
At power up, Fuse/39, you may hold down the Amp/3 pushbutton to give a graceful, no-thump power-on, partly from resistors limiting 15000uF cap charging and partly from putting 8 big transistors near ground.
Protection circuits: Amp/7 detects loss of one of the supplies & puts the output transistors near ground. S/3 monitors the 8 big transistors for shorts, if so then the output transistors are sent to ground. Speaker/Woof monitors the power going to the subwoofer's voice coil, especially at low frequency where the natural cooling of the Elevated Frame isn't pushing much air for cooling, & clicks in a relay to attenuate what gets to the speaker. A lot of protection is warranted when $225 of speaker is at stake.
Oil temp indicator at Amp/Woof is actually on an output transistor, in case you forget to put in the mineral oil.
The 6x9" midrange speakers can't handle the same power level as subwoofer. Provide warning LED at Speaker/Mid but not automatic protection.
Subwoofer cone at Speaker/Woof has a note, "+ and - cone motion sensor & LED." This is some sort of (probably) optical sensor for the rated Xmax travel, +-12mm. Warning LED. Xmax violation is not yet damage, it is onset of increasing distortion as voice coil starts leaving the magnet gap. Optical sensing is not too big an engineering challenge, it will probably be with aid of a paper flag glued lightly onto cone. LED/phototransistor can be arranged as optical interrupter, but positioning them is an opportunity for 3D printer at A.R. Johnson HS. Also, a cap for oil reservoir to keep ice out, if the oil reservoir is surrounded by an ice bath.
Reading in Bob Cordell's power amplifier book, I find that the emitter resistors at the output transistors need to have .026V across them, that is supposed to give the lowest harmonic distortion during crossover between NPN & PNP. If quiescent current is .1A there, the emitter Rs are .26ohm each, but power might need to be 14W for the dual-resistor package.
At Amp/Woof, an easy & neat addition to the Class AB woofer amplifier is the ability to convert it to Class A. This can be done with a three-2N3055 current source down to -24V, with SPDT switch to change between A and AB. The question is whether the Class A setup should be non-feedback. If non-feedback, the Darlington output transistors could be simply driven by OPA551. The current source that gives "negative" speaker current is active, also, on positive peaks, meaning that the NPN transistors have to deliver twice the current on peaks. This is the guide for deciding the current source amps, maybe 3A.
At Amp/Woof, soft clipping might be optoisolator sensing voltage across output transistors with the LED, then the phototransistor can act to limit current to base of Darlington output. When voltage available to the output transistor falls to about 2V, base drive starts drying up & it soft clips.
The THD row is an attempt to measure total harmonic distortion at a single frequency, the 65.4Hz of the reference sine. This assumes the 65.4Hz sine is clean enough, there may be a need to have additional bandpass filter of 65.4Hz to make it cleaner. THD is a key point in Bob Cordell's book, Designing Audio Power Amplifiers.
DC Power Supply Outline
31
35
39
43
47
51
Line
Cord
permanently wired cord
Filter
+-15V series regulators to power DIY 2012 stereo amplifier are in the power-supply box
line filter; relay & inrush thermistor for each supply, delay turn-on for -24V by .4s
Fuse
lamp, fuse
DC
+24V commercial supply 8.3A,
Jameco 668730 200W, $55 each, inrush limit by Epcos 5ohm (when cold) 4.5A thermistor
-24V commercial supply 8.3A, inrush limit by thermistor
7.5V 2A wall-pack supply for 5V logic & headphone amplifier
5.15V 1A wall-pack supply for negative headphone amp supply
16V aux supply 3.7A regulated to 12V for relays, sale $3
Meter
ammeter, lamp
ammeter, lamp
lamp
lamp
lamp
Diode
output diode to prevent reverse polarity damage
same
LC
LC filtering to isolate switching noise
same
Conn
barrier strip
bleeder resistor
bleeder
in safety enclosure so there is no fire hazard, no shock hazard
input 120VAC, convenience outlets for CD player
Notes
The 120VAC, from the wall outlet, is a potentially lethal voltage. The warning at Line/35 must be carried out in protective hardware and labeling so no one gets shocked. In addition, the +-24V DC supplies have enough voltage that a person touching +24V with one hand and -24V with the other hand has 48V across the body; this is also dangerous, but label warnings are adequate. Student research option: http://en.wikipedia.org/wiki/Electric_shock
Lamp at Fuse/39 is a neon lamp on AC line, plus LEDs on +-24V.
Ammeters at Meter/35 and Meter/39 are to let power consumption be measured.
Diodes at Diode/35 and Diode/39 are several, paralleled silicon diodes to keep other power supplies from being accidentally applied in reverse to the outputs and blowing up the capacitors in the supplies. Example: +24V output goes to anode, then cathode goes on to the loads. In the amplifier, not in the power supply enclosure, put several Jameco 15000uF 50V for bypassing at the amplifier; these are just $4.49 each in 10 qty, but check Marlin P. Jones for better price.
LC filtering at LC/35 is because the 30kHz or 150kHz switching in the open-frame supplies amounts to RF noise and must be bottled up in the metal supply enclosure.
Low-level Circuits Description
Additional Details of Woofer Amplifier
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